THE EYE AND VISION. 



121 



Place a clear, strong solution of chrome alum in a glass 

 bottle with flat sides. Close the eyes for a few minutes 

 and then look through the solution when held between 

 one eye and a white cloud. A rosy spot should be seen 

 in a green field of vision. The pigment in the yellow 

 spot on the retina absorbs the blue-green rays, and so the 

 remaining rays which pass through the alum give a pink 

 color. 



J. Irradiation. 



FIG. 13. 



a. In Fig. 13 look at the white spot on the black 

 ground, and at the black spot on the white ground. 

 Which spot appears larger? 



b. In Fig. 14 look at the four squares, two white and 

 two black. Which looks larger, the white or the black? 

 What is meant by irradiation? 



K. Imperfect Visual Judgments. 



a. Make three round black dots, 

 1, 2, and 3, of the same size and in 

 the same line, so that 1 and 3 are equi- 

 distant from 2. Make several other 

 dots of the same size between 1 and 2. 



Which now appear to be farther apart, 1 and 2 or 2 and 3 ? 



b. Make two squares of equal size with very light out- 



FlG. 14. 



